This invention relates to a method for producing metal or alloy articles having complicated configurations with high precision by mixing and kneading a metal or alloy powder with an organic binder and other materials, molding the mixture into a shape similar to the objective product by injection molding, removing the binder and other unnecessary materials, and then compacting the molding by sintering.
It is well known to produce metal or alloy articles by powder metallurgy in which a metal or alloy powder is molded by suitable molding means such as press molding, CIP molding, etc., and the molding is compacted by sintering to obtain the objective article having the desired configuration and properties. Conventional molding techniques employed in such powder metallurgy, however, have the problems of their own. For instance, in the case of press molding, it is unable to produce articles of other shapes than those obtainable by uniaxial molding. CIP molding, although capable of molding articles of three-dimensional configurations, has problems of poor precision and unsuitableness for mass production since the molding is conducted in a rubber mold. Owing to the progress in the field of injection molding of plastics in recent years, however, the techniques have been developed for molding articles of complicated configurations with high precision by mixing a metal or alloy powder with plastic material and injection-molding the mixture. The plastic material used as a binder is removed after molding by thermal decomposition or other chemical means, followed by sintering of the molding for compaction thereof. In this process, the moldability and binder removability are greatly affected by the selection of the binder used.
Several technical disclosures have been made regarding the binder useful in the metal or alloy article production process in which a metal or alloy powder is injection-molded and then compacted by sintering. For instance, Japanese Patent Publication No. 29170/76 discloses an injection molding composition made by blending a ceramic material, a lubricant such as atactic polypropylene, wax, paraffin, etc., and a plasticizer such as diethyl phthalate. Japanese Patent Laid-Open No. 113511/80 proposes mixing of a thermoplastic resin and a silane or titanium coupling agent with a ceramic or metal powder for injection or extrusion molding. Japanese Patent Laid-Open No. 229403/84 discloses many binders, typically a binder for injection molding in the production of sintered metal articles, said binder having a composition comprising 30-50% by weright of at least one of ethylene-vinyl acetate copolymer and low-density polyethylene, 19-32% by weight of methacrylic acid ester copolymer, 7-13% by weight of at least one of dibutyl phthalate, diethyl phthalate and stearic acid, and a balancing amount of paraffin wax.
In any of these disclosures, a thermoplastic or thermosetting resin generally called plastic is used as a binder and suitably mixed with a plasticizer, lubricant and/or other necessary materials, and usually the binder is used in an amount of 50% by volume ratio to the powders of raw materials, which corresponds to 8-25% by weight. Such a binder is removed after molding by making use of thermal decomposition of plastic in an oxidizing atmosphere in the case of ceramic and in a non-oxidizing atmosphere in the case of metal powder. In such binder removal operations, in order to prevent cracking or creep deformation of the molding, the heating rate has to be controlled usually below 20.degree. C./hr, so that a long time of more than 40 hours, even reaching 100 hours in some cases, has been required for the binder removal. Thus, the conventional techniques have necessitated the industrially impractical number of steps and energy consumption. Techniques have been proposed for chemically removing the binder by using an organic solvent, but in this case, the molding becomes delicate and difficult to handle. Further, there has been a problem of the elevated product cost because of the use of costly plastic resin in an amount of more than 10% by weight and the inability to recover such resin at the time of binder removal.
With these problems, injection molding has been scarcely applied to practical use for the production of ceramics and metal articles in spite of its generally recognized advantages.
On the other hand, U.S. Pat. No. 4,113,480 discloses the use of a water-soluble binder. According to this U.S. patent, methylcellulose is selected as an organic binder which is lowered in the solubility for the solvent increases in viscosity at high temperatures compared with room temperature. To a Co-based super alloy gas atomized powder having a particle size passing a 325 mesh sieve, 1.5 to 3.5% by weight of methylcellulose, 0.25 to 2.0% by weight of glycerin, 0.1 to 1.0% by weight of boric acid and 4.0 to 6.0% by weight of water are added and mixed, followed by injection molding. Glycerin is added as a molding aid and boric acid is added as a sintering accelerating agent. The mold temperature of injection molding is 170.degree. to 190.degree. F. (77 to 88.degree. C.). According to this patent, there are advantages in that the adding amount of binder is smaller than that of a prior art thermoplastic resin binder, and the removal rate is faster than the prior art thermoplastic resin binder. But there are the following disadvantages in that (1) the use of only cellulose and glycerin makes the molding properties worse and the kneaded mixture is not filled completely in fine portions of the mold, and (2) the use of boric acid is effective for accelerating the sintering due to lowering in the sintering temperature, but is not preferable because of remaining in the sintered body in an amount of 0.05 to 0.30% by weight to make the sintered body remarkably brittle. Particularly in the case of an iron-based carbon-containing material, there is produced an iron-boron compound which is hard and brittle.